maximum gap between toilet flange and floor

I am in the process of remodeling two bathrooms. I'm planning to replace the tile flooring with vinyl. For one of the toilets, I can easily replace the flange by cutting the pipe from the floor below. The second toilet is not so easy. I don't have easy access to the waste pipe from underneath because it is directly above a heating duct. It also looks like the flange is connected directly to an elbow. The drain pipe is 3" inside diameter. I understand it is preferred to have the flange resting directly on top of the floor, but it would require either taking a heat duct apart or pulling up the subfloor. So my question is if I tear out the floor and put down new underlayment and vinyl, I'll have a 1/8" gap between the flange and floor. Is this acceptable, or will I need to shim the flange and toilet? And if I shim the flange will the toilet need to be shimmed as well or would it still be possible to push it down tight to the floor?

Put a shim underneath the flange so that it fills that gap, and then rescrew it to the subfloor. You might be able to use a few washers at the screw holes, but a solid ring may be better. Being 1/8" higher than design rarely would make it too high for the toilet to sit properly, but make sure the toilet DOES sit flat over the flange before setting it with the new wax ring. There has to be at least some room for wax between the toilet and the flange for it to seal properly. And, if the toilet rocks, add shims there to prevent it.